Hi again. For those that have issues with the front master cylinder on their GL1000, I hope to be able to provide a little helpful info.. Not too long ago a buddy helped me rebuild the front brake master cylinder on my '76. The brakes worked fine, but brake fluid was leaking slowly from the area of the piston, where the lever presses on the "plunger". The piston and bore were fine. He/we did some homework and were able to obtain "o-rings" and "cups" locally that work perfectly.

I am going to go back to the sources for my parts to obtain their new/updated part numbers because the pn's on my paperwork do not appear to be valid as of now.

I realize the importance of safe, effective brakes, and concur that you should never "cheap-out" on brake systems. However, if someone needs to freshen up the master cylinder, ie the piston, bore, spring, etc are ok, or someone is on a tight budget, if you can get by with a cleanup and just replacing the "o-ring" and cup, I will be providing info on these as soon as I can get back to the local shops. The cups are name brand automobile parts, and the "o-rings" are actually X-rings, made of Viton, and are ultimately from an industrial source. Stay tuned...

In my case, regarding a 1976 GL1000, the front brake master cylinder piston cup seal is available from Auto Value parts stores (at least). The PartsMaster (manufacturer) part number is WCC2502 and is called a "wheel cylinder cup". The price today at the store I checked is $5.09 and is for a pack of 10. They don't normally stock the part but can have it in about 2 days. Kind of worth the wait I think.. They didn't have a cross reference part no.

As for the o-ring or X-ring, I ordered mine from Motion Industries in Lansing, MI. They are a typical bearing and seal supplier. I have ordered wheel bearings, etc from them. They would have to order these. Their part number is VQ4205 and they refer to it as a Viton Quad Ring. The price was $2.98 ea plus shipping back in 2012. Some specs I was able to come up with from MInnesota Rubber: their part no is 4205-514AD4 = Seal type ie quad or x-ring205 = Ring size514AD = Compound ie VitonBore: .687 in I.D.: .421 in 10.69 mm Cross section: 0139 in 3.53 mm

WingAdmin says these most likely will fit many other Honda models and years.

This is a bit of an older thread but I am looking for an o-ring to replace the one on my GL1000 front M cylinder reservoir. While researching a suitable replacement I found charts that advise viton is not good with DOT 3 type fluid. How are these holding up? From the compatibility charts I am seeing EPDM or butyl is the preferred material. Hog wash?-G

Thanks for bringing this up. My mechanic buddy that helped me with this "project" said they used Viton at his work (Oldsmobile/BOC/GM) for many applications and was their main choice. That's why he recommended it. I did quick some Googling and but didn't see a specific yay/nay answer other than a blogger that stated that "Viton was not recommended". He may be very knowledgeable, but didn't state any data. This site http://www.marcorubber.com/compatibility.htm says "Insufficient data".

When questioned about this, my buddy said he had not heard these concerns, but that even if in fact DOT 3 causes it to swell, wouldn't that be a good thing? He said that the main concern would be deterioration. At any rate, I can not report experience other than that I have not had any trouble with my bike, although it has only been three years and about 4,000 miles.

Bottom line, until any of our members can speak as an expert here, all I can say is that perhaps readers should hold off using these if they are uncomfortable with it.

The nearest equivalent that I have found is O-rings in the -132 size. They are available in various materials including brake fluid friendly ones. I have not determined where I'm going to buy them yet at least I've got a target. -G